Correct pre-charge pressure of the expansion tank – how to set and check

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Precharge pressure of expansion tank – why it is one of the most important, yet most often neglected settings in heating

When an installer hands over a new heating system, most customers ask about boiler performance, gas consumption, and radiator temperatures. Rarely does anyone ask about the expansion tank and its precharge pressure. And yet, this seemingly insignificant parameter determines whether the system will operate reliably for years without problems, or whether it will be air-locked every morning, the boiler will lose pressure, the safety valve will drip, and the membrane will burst literally before its time.

In practice, I have encountered dozens of cases where the customer called saying that "the boiler is losing pressure" or "I have to top up the water every month." Upon arrival at the site – the precharge pressure of the expansion tank was set incorrectly or completely discharged. Solution: 10 minutes of work with a pressure gauge and a pump, and the system functions without intervention for the next several years.

This article will explain what precharge pressure actually is, why it occurs, how to correctly determine it for your specific system, how to check and set it, and what happens if you neglect it. If you are looking for more basic information about selecting the correct volume or comparing types of tanks, see the articles What expansion tank volume do I need for my heating system and Membrane vs. conventional expansion tank – differences and when to use which in the same Knowledge Center.

What is the precharge pressure of an expansion tank and how does the membrane work

An expansion tank is a pressure vessel divided by a flexible membrane (or bladder) into two chambers. One chamber is the air chamber – filled with nitrogen or dry air under a defined pressure. The other chamber is the water chamber – connected to the heating circuit. When the water in the system heats up and expands, it flows into the water chamber and compresses the air on the other side of the membrane. When the system cools down, the air pushes the water back.

Precharge pressure (in English "pre-charge pressure", in German "Vordruck") is the air pressure in the dry chamber of the tank in the state when the water chamber is empty – that is, before the system is filled with water. It is a basic parameter from which the entire behavior of the tank during operation depends.

The precharge pressure must be correctly set before the tank is installed in the system and before the system is filled with water. Most manufacturers, including IBO expansion tanks, deliver tanks with a precharge pressure of 1.5 bar from the factory. This is, however, only a "standard" value that suits many, but not all, installations. If you have a system with a higher static pressure (e.g., a multi-story house), it is necessary to adjust the precharge pressure.

How to correctly calculate the precharge pressure for your system

The precharge pressure of the expansion tank must be equal to or slightly higher than the static pressure of the heating system at the point of installation of the tank. Static pressure is the pressure of the water column between the lowest point of the system (expansion tank) and the highest point of the system (usually the last radiator or the highest point of the piping).

The calculation is simple:

Measure the height difference (H) in meters between the point of installation of the expansion tank and the highest point of the system.
Every 10 meters of water column corresponds to 1 bar of pressure.
Precharge pressure = H (m) ÷ 10 + reserve 0.2–0.3 bar

Practical examples:

Single-story house – boiler in the technical room, the highest radiator 2.5 m above the tank: static pressure = 0.25 bar → precharge pressure is set to 0.5 bar (standardly not below 0.5 bar anywhere due to safety reserves)
Two-story house – highest point 5.5 m: static pressure = 0.55 bar → precharge pressure 0.75 – 0.8 bar
Three-story house or family villa – highest point 9 m: static pressure = 0.9 bar → precharge pressure 1.1 – 1.2 bar
Apartment building or building with a boiler room in the basement – highest point 20 m: static pressure = 2.0 bar → precharge pressure 2.0 – 2.2 bar

Important rule: the tank precharge pressure must NEVER be lower than the static pressure of the system. If it were lower, the membrane would be pushed out to the air valve immediately when filling the cold system, and the tank would not perform its function – the entire volume would be filled with water without any reserve for expansion.

Working system pressure vs. pre-charge pressure of the tank – how they relate

Here is another important relationship that needs to be understood. Every closed heating system has:

Filling (cold) pressure – the pressure at which the system is filled with cold water. Typically 1.0 – 1.5 bar in family homes.
Operating (hot) pressure – pressure at full operating temperature (70–90 °C). Typically 1.5 – 2.5 bar.
Maximum pressure – the setting of the safety valve, usually 3 bar in domestic systems.

The pre-charge pressure of the expansion tank must be set to be the same or at least equal to the filling pressure of the system. Why? Because when filling with cold water, we do not want water to enter the tank at all (the tank is then at rest, the air layer is pressing equally or with greater force than the water circuit). Only after heating and expansion of the water will water start to enter the tank and compress the air.

If the pre-charge pressure is lower than the filling pressure, water will flow into the tank immediately during filling – even before heating. This reduces the effective volume available for expansion and may result in the tank not being able to accommodate the entire volume of expanded water when operating temperature is reached. Result: the safety valve opens.

If the pre-charge pressure is too high, the membrane pushes back against the water and the system may have a problem with low pressure – the boiler will report a low pressure fault, or you may need to constantly top up the water.

Conclusion: pre-charge pressure = system filling pressure (or 0.1–0.2 bar less, not more). This is the golden rule.

How to check the pre-charge pressure – step-by-step procedure

Checking the pre-charge pressure is a simple operation that any handy homeowner can perform. You only need a standard tire pressure gauge (or a more precise manometer) and possibly a pump.

Procedure in detail:

Turn off the boiler and let the system cool down – pre-charge pressure measurement must be performed on a cold system (up to 30 °C). On a hot system, the water in the tank is under pressure and measuring the air chamber would be distorted or impossible (water is pressing against the membrane and prevents accurate reading).
Close the connecting valve of the expansion tank (if it exists) and release water from the water chamber – either via the system's bleed valve or by reducing the system pressure using a drain valve. The goal is to bring the water pressure down to zero (or at least below the air pressure), so the membrane is not pressing against the air side.
Unscrew the cap of the air valve – most membrane-type expansion tanks have a standard valve on the air side (usually at the top of the tank or on the side opposite the connection).
Attach the pressure gauge and measure the air pressure. A standard tire pressure gauge is sufficient, but for more accurate measurement, I recommend a digital manometer with a precision of 0.1 bar.
Compare the measured value with the required one (calculated or according to the project).
If the pressure is low – add air using a standard pump. If it is high – release air through the valve (using a thin object, e.g., a pin or a nail).
After setting the correct pre-charge pressure, cover the valve with the cap, open the connecting valve, and refill the system to the correct filling pressure.

How to set the pre-charge pressure – practical tips from the field

From practical experience, I give you a few tips you won't find in manuals:

Never measure on a hot system. I have seen this many times – a customer measures the pre-charge pressure while the boiler is running, reads 2.5 bar and asks why it is so high. Because the membrane is compressed by hot water. After cooling down, it would show 0.8 bar. Always use a cold system.

Use a quality pressure gauge. A cheap pressure gauge from a gas station may have an error of ± 0.3 bar, which is a huge difference when setting a value of 0.75 bar. A digital manometer for 15–20 € is an investment that pays off.

Do not install the tank without verifying the pre-charge pressure. New tanks, including the IBO 8l expansion tank, IBO 12l or IBO 19l, come from the factory with a pre-charge pressure of 1.5 bar. If your system requires 0.75 bar, you must reduce the pre-charge pressure before installation. If it requires 2.0 bar (e.g., an apartment building), you must increase it. Never assume that the factory pre-charge pressure is correct for your specific case.

After setting the pre-charge pressure, always check the valve for tightness. Simply apply soapy water to the valve. If bubbles appear, the valve is leaking and air will gradually escape. Solution: replace the valve insert (standard automotive/bicycle insert).

Check the pre-charge pressure once a year. A good habit is to check the pre-charge pressure at the beginning of the heating season – ideally together with the boiler inspection. Over the course of a year, the pressure can drop by 0.1–0.3 bar even with a perfectly tight valve – air is a gas and micro-leaks are common.

Recommended pre-charge pressure table according to building height and filling pressure

Building type / height	Static pressure	System filling pressure	Recommended pre-charge pressure
Single-story house (max. height 3 m)	0.3 bar	0.8 – 1.0 bar	0.5 – 0.8 bar
Two-story house (height up to 6 m)	0.6 bar	1.0 – 1.2 bar	0.8 – 1.0 bar
Three-story house (height up to 9 m)	0.9 bar	1.2 – 1.5 bar	1.0 – 1.2 bar
Building / apartment block (height up to 15 m)	1.5 bar	1.8 – 2.0 bar	1.5 – 1.8 bar
High-rise building (height up to 25 m)	2.5 bar	2.5 – 3.0 bar	2.3 – 2.5 bar

What happens if the pre-charge pressure is incorrect – symptoms and consequences

Incorrect pre-charge pressure has very specific and detectable symptoms. It is good to know how to recognize them, so you can tell when it is time to check the expansion vessel.

Symptoms of too low pre-charge pressure (or depleted air)

Pressure relief valve drips or sprays during heating – the vessel is filled with water without air, so there is nowhere for the water expansion to go and pressure rises above the maximum level.
Boiler indicates low pressure after cooling down – when cooling, water contracts, but the vessel cannot push it back (the membrane is pressed against the valve, air has no pressure). System pressure drops.
Pressure fluctuates repeatedly – low in the morning, high during the day – a classic sign of a faulty expansion vessel (either depleted or ruptured membrane).
Need to regularly add water – the system "drinks" water, even though there is no visible leak. Water escapes through the pressure relief valve, and you add it back through the filling unit.

Symptoms of too high pre-charge pressure

Boiler indicates low pressure even after fresh filling – pre-charge pressure is higher than the system filling pressure, the membrane pushes water from the vessel into the circuit and the pressure gauge shows a lower pressure than it should.
System is difficult to fill to the desired filling pressure – air flows from the vessel into the system through the membrane during draining (ruptured membrane + high pre-charge pressure).
Heating is cold, even though the boiler is running – extremely rare, but in case of a completely failed vessel and incorrect pressure, an air lock may occur.

If you recognize any of these symptoms, I recommend reading the article Common expansion vessel faults – symptoms, causes and solutions, where these scenarios are described in more detail, including diagnostics to determine whether the problem is with the pre-charge pressure or a ruptured membrane.

Ruptured membrane vs. depleted pre-charge pressure – how to distinguish them

This is a question I get almost with every service call. The customer knows something is wrong with the expansion vessel, but does not know whether to re-pressurize it or replace it.

The procedure for distinguishing them is simple:

Turn off the boiler and let the system cool down.
Reduce the pressure in the system (drain water), so the water pressure is at zero.
Attach a pressure gauge to the air valve of the vessel.
If air comes out of the valve and the pressure gauge shows a value (low, e.g. 0.1–0.3 bar) – the membrane is likely in good condition, only the pre-charge pressure is low. Re-pressurize and the problem is solved.
If water or water vapor comes out of the valve – the membrane is ruptured. Water has passed into the air chamber. The vessel must be replaced.
If nothing comes out of the valve (pressure gauge shows 0 bar) and the valve is tight – the membrane is likely ruptured and water has filled the entire interior including the air chamber. The vessel must be replaced.

When replacing the vessel, we recommend always considering a larger volume – especially if the system has added new consumers (new storage tank, solar panels, larger floor area). For larger systems, we also offer IBO 24l and IBO 36l, which are ideal for houses with a larger water volume in the circuit or with TÚV systems. More about the correct volume selection can be found in the article What expansion vessel volume do I need for my heating system.

Specific situations – TÚV, solar, floor heating

Pre-charge pressure in TÚV (domestic hot water) and solar systems is a different chapter. These systems have higher pressure and higher operating temperatures, which also affects the setting of the expansion vessel.

TÚV systems

In hot water preparation systems (tanks, boilers), temperatures reach 60–85 °C and the water expansion volume is greater. In addition, water pressure in households is usually 2.5 – 4 bar (depending on the location and presence of a pressure-reducing valve). The pre-charge pressure of the expansion vessel for TÚV must match this water pressure – if you have 3 bar in your home, the pre-charge pressure of the TÚV vessel must be at least 3 bar. More about this specific case can be found in the article Expansion vessel for TÚV systems – what it must meet and how to choose it.

Solar systems

Solar circuits operate with glycol and reach extremely high temperatures (up to 150–180 °C in stagnation). The pre-charge pressure of the solar expansion vessel is set higher – usually 1.5 – 3 bar depending on the height and configuration of the system. For solar applications, special vessels with more durable membranes resistant to higher temperatures and glycol media are used.

Floor heating

Floor heating has a larger water volume in the pipes (large amount of tubing in the concrete slab) and lower operating temperatures (35–45 °C). The expansion volume is relatively small, but the total system volume is larger. Pre-charge pressure is set the same way according to the system height, but the focus is on the correct vessel volume – for floor heating, a larger vessel is needed than for a radiator system of the same heating area.

Nitrogen vs. air – what to use for filling the expansion vessel

The majority of standard expansion vessels for domestic heating are pre-filled with dry air or nitrogen. For standard domestic heating applications (media temperatures up to 90 °C, membrane made of isobutylene or EPDM), air from a bicycle pump is completely acceptable for topping up the pre-pressure.

For solar systems and high-temperature applications, filling with nitrogen (N₂) is recommended. The reason is simple: air contains oxygen and moisture. Oxygen can cause corrosion of metal parts of the vessel at high temperatures, and moisture can damage the membrane from the inside. Nitrogen is inert and safe.

For standard checks and topping up the pre-pressure in domestic heating, a standard bicycle or car workshop pump is sufficient. You will use nitrogen only if the vessel manufacturer explicitly requires it or if it is a solar or high-temperature circuit.

How long does the set pre-pressure last and when to intervene

Under normal conditions, the set pre-pressure should last several years without the need for intervention. In practice, however, we observe a gradual drop – usually 0.1 – 0.3 bar per year in a properly functioning vessel. This is natural.

If the pre-pressure drops faster (e.g., 0.5 bar per season), look for the cause:

Leaky valve seal – replace (standard automotive valve seal).
Cracked valve gasket – seal or replace the valve.
Micro-crack in the membrane – water slowly enters the air chamber, it is time to replace the vessel.

The golden standard is to check the pre-pressure once a year – ideally before the heating season. This simple check can prevent all the problems described above. More about the lifespan of vessels and when to replace them can be found in the article How long does an expansion vessel last and when to replace it.

Where to buy and what to focus on when choosing a vessel

When choosing a new expansion vessel (whether as a replacement or for a new system), I recommend focusing on three things: volume, maximum pressure, and membrane quality.

For standard family homes (water content in the system 50–150 liters), vessels in the range of 8 – 24 liters are ideal. IBO 8l is suitable for small systems (small apartment, studio, ground-floor cottage with a few radiators). IBO 19l covers most standard family homes with central heating. For larger homes or systems with floor heating and/or domestic hot water storage, I recommend IBO 36l, which has sufficient volume for larger installations.

All IBO vessels have a maximum operating pressure of 6 bar and a working temperature up to 99 °C, making them suitable for standard heating. The membrane is made of EPDM rubber – heat-resistant and chemically resistant. The connection thread is G¾", a standard compatible with most boilers and fittings on the market.

Frequently asked questions (FAQ)

What pre-pressure should I set if I don't know the system height or filling pressure?

In practice, if you have no information and it is a standard family home (1–2 floors), set the pre-pressure to 0.75 bar. This is a safe middle value that suits most two-story homes with a filling pressure of 1.0 – 1.2 bar. Then, when filling the system, monitor the pressure to which the cold system fills – that is your filling pressure and the pre-pressure should be the same or 0.1–0.2 bar lower.

Can I set the pre-pressure when the vessel is already installed in the system?

Yes, but the system must be cold and the water chamber must be drained (water pressure in the system zero or minimal). If you don't have a shut-off valve before the vessel, you must drain the entire system. Therefore, professionals always recommend installing a service valve on the vessel connection – it is a small investment that saves time with every future check.

What happens if I set the pre-pressure too high – for example, 3 bar in a system with a filling pressure of 1.0 bar?

The vessel will be practically non-functional. The air inside will press on the membrane with a force of 3 bar, while the water in the system has a pressure of 1.0 bar. The membrane will be pressed on the water side and the water will not be able to enter the vessel. The entire water expansion will be handled only by the safety valve – it will open with every heating cycle. In addition, when cooling, the pressure in the system will drop far below the minimum. Result: the safety valve drips, the boiler reports a low-pressure fault.

Can I use an expansion vessel from heating for a TÚV system and vice versa?

No, without verification. Vessels for TÚV must have a membrane certified for contact with drinking water (NSF/WRAS certification or equivalent). Standard heating vessels do not have this certification – their membrane may release substances unsuitable for drinking water. In the opposite direction (TÚV vessel in heating) it is technically possible, but unnecessarily expensive. Always use vessels intended for the specific application.

Why does my pre-charge pressure drop by 0.5 bar every year, even though the vessel is relatively new (2 years old)?

The most common cause is a leaking valve core. Air escapes around the valve core (same as in tires) and is slowly lost. Solution: buy a set of valve cores (a few cents at an auto shop) and replace the core. Before replacement, release the remaining air. After replacement, inflate to the correct pre-charge pressure and check the seal with soapy water. If the leak continues after replacing the core, it may be a micro-crack in the vessel body or membrane – in that case, it is time to replace the entire vessel.

How can I determine the pre-charge pressure if I lost the documentation from the boiler/system?

You are not dependent on documentation. The pre-charge pressure can always be calculated from physical parameters: measure the height difference between the vessel and the highest point in the system (radiator on the top floor or air vent). Every 10 meters = 1 bar of static pressure. Set the pre-charge pressure to the static pressure + 0.2 bar. At the same time, check the filling pressure of the system (read from the manometer when the system is cold after filling) – the pre-charge pressure must not exceed this filling pressure.

Conclusion – Pre-charge pressure is a five-minute job that can save you years of problems

Correctly set pre-charge pressure in an expansion vessel is a fundamental requirement for the reliable and trouble-free operation of every closed heating system. It is not a scientific operation – you need a pressure gauge, an air pump, and 10 minutes of attention. On the other hand, neglecting this parameter leads to real damage: cracked membranes, open safety valves, water loss from the system, boiler warnings, and in the worst case, boiler or pipe damage.

Basic rules to remember: pre-charge pressure = system filling pressure (or slightly less, never more), always measure on a cold system, check once a year, never estimate – always measure. And if you are unsure about the size of the vessel, its location, or the overall system setup, check other articles in this Knowledge Center – for example, Installation of an expansion vessel for heating – procedure, placement and connection or the overview IBO expansion vessels – model overview and parameter comparison.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.